Trigeminal nucleus caudalis is known to be essential for the perception of noxious facial stimuli. Some have suggested that this function relates to the unique location of a population of nocispecific neurons in nucleus caudalis. Restricted populations of nocispecific neurons have been identified in nucleus caudalis under rigid experimental conditions in several recent studies. The possibility that nocispecific neurons may be located in other portions of the spinal trigeminal nucleus has not been excluded. It appears impossible, however, to explain many observations concerning pathological pain states on the basis of strict specificity theory. Others have suggested that nucleus caudalis may act as a modulator of information transmission through multimodal cells located in more rostral portions of the spinal trigeminal nuclei. The proposed research will employ post stimulus time histogram analysis of activity elicited in neurons located in rostral and caudal portions of the spinal trigeminal nuclei in response to a variety of natural noxious and non-noxious stimuli. An attempt will be made to modify activity recorded in rostral neurons by varying the neural input from nucleus caudalis to those rostral areas. The anatomical substrate for such modification is well established. We feel that the relay of "leminiscal" or "spinothalamic" information at the rostral or caudal subdivisions respectively of the spinal trigeminal nucleus is an outmoded concept. Many of the paradoxical features of facial nociperception as well as pathological facial pain states may be clarified by an understanding of the mechanisms of interaction of the rostral and caudal subdivisions of the spinal trigeminal nucleus. BIBLIOGRAPHIC REFERENCES: Young, R. F. and Nord, S. G. Experimental modulation of medullary dental pulp units by mechanical stimulation of oro-facial fields. Exp. Neurol. 49: 813-821, 1975.